Calculating the number of days between two dates in JavaScript (the right way)

FontFiesta — Wed, 12 Aug 2026 20:37:55 +0000

"How many days until the deadline?" sounds trivial until you actually write the code and get bitten by time zones, daylight saving time, and off-by-one errors. Here's a clear, correct way to do it.

The naive version (and why it's slightly wrong)

The classic one-liner subtracts two Date objects and divides by the number of milliseconds in a day:

const msPerDay = 1000 * 60 * 60 * 24;

function daysBetween(a, b) {
  return Math.round((b - a) / msPerDay);
}

daysBetween(new Date("2026-01-01"), new Date("2026-03-01")); // 59

This works most of the time, but it silently breaks across a daylight-saving boundary, where a "day" is 23 or 25 hours long. Math.round hides the error for short ranges and produces off-by-one results for longer ones.

The correct version: normalize to UTC midnight

Strip the time component by converting both dates to UTC midnight before subtracting. Now every day is exactly 24 hours:

function daysBetween(a, b) {
  const utcA = Date.UTC(a.getFullYear(), a.getMonth(), a.getDate());
  const utcB = Date.UTC(b.getFullYear(), b.getMonth(), b.getDate());
  const msPerDay = 1000 * 60 * 60 * 24;
  return Math.round((utcB - utcA) / msPerDay);
}

Because we've zeroed out hours, minutes, and seconds, DST no longer matters — the subtraction is always a whole number of days.

Inclusive vs. exclusive counting

One question trips people up: does the count include both endpoints? "From the 1st to the 3rd" is 2 days if you count gaps, but 3 days if you count calendar days touched. Decide which your feature needs and add + 1 for the inclusive version. Being explicit about this in your UI saves a lot of support tickets.

Don't reinvent it every time

For a quick answer without writing code, I use CalcPine's days between dates calculator — it handles the inclusive/exclusive distinction and shows the breakdown in years, months, and days, which is handy for sanity-checking your own implementation.

Reaching for a library?

If you're already pulling in a date library, date-fns has differenceInCalendarDays(b, a) and Luxon has DateTime.diff. Both normalize calendar days for you. But for a single count, the UTC-midnight trick above is a dependency-free four-liner worth keeping in your snippets folder.